Stainless steel and method



May 2, 1950 G. N. GOLLER 2,505,762

STAINLESS STEEL AND METHOD Filed Sept. 6, 1946 GEORGE Y N. GOLLER wwf/JQ@ rf Patented May 2, 1950 s'rAmLEss STEEL AND METHOD George N. Goller, Baltimore, Md.,

Armco Steel Corporation,

assignor to a corporation of Ohio Application September 6, 1946, Serial No. 695,215

(Cl. 'l5-124) 11 Claims.

The invention relates to stainless steel, lmore I particularly to the provision of chromium-nickel stainless steel in hardenable and hardened conditions, and to the resulting products and manufactures.

Among the objects of my invention is a direct, reliable and highly effective method for providing chromium-nickel stainless steels in a hardenable yet soft workable condition suitable for a wide variety of forming and fabricating operations, for example those including cold-rolling, drawing, stamping, punching, upsetting, or machining, and in a strong hardened condition by subsequent heat `treatment without substantial warping or heat-scaling at the hardening temperatures employed.

Other objects of my invention in part will be obvious and in'part pointed out hereinafter.

The invention accordingly consists in the combination of elements, composition of materials and features of products, articles and manufactures, and in the various operational steps, and the relation of each of the same to one or more of the others as described herein, the scope of the application of which is indicated in the following claims.

The single figure of the accompanying drawing graphically represents proportions of chromium and nickel which are at times employed in the composition of my stainless steel, as will be pointed out more fully hereinafter.

As conducive to a clearer understanding of certain features oi my invention, it may be noted at this point that the better known chromiumnickel stainless steels, as for example the 18--8 chromium-nickel steels, are stably austenitic at room temperatures and under no circumstances respond to hardening heat treatment. The steels are work-hardenable, but in actual practice there are rather sharp limitations upon the possible utilization of this property to achieve desired hardness. Usually work-hardened products of the steel are under strain, and the customary annealing treatment to relieve the strain tends to reduce the hardness. Nevertheless, there are many valuable properties of the steels including cold workability as by rolling, swaging, cold heading, bending or drawing. They are machinable and possess many additional features, such as excellent corrosion resistance, which create a very considerable demand for their use.

The chromium-nickel stainless steels, as for example those of the varieties just noted, irequently include additions of such elements as copper, manganese, silicon, cobalt, molybdenum,

tungsten, vanadium, titanium, columbium, and sulphur, for special purposes. Some few of the chromium-nickel stainless steels have been known to respond to hardening heat treatment, this by virtue o1' the inclusion of titanium or columbium in well studied proportionment with other elements present, coupled with a critical form of heat treatment to effect precipitation hardening. The chromium-nickel, titanium or columbium stainless steels, however, are expensive to produce in view of the relatively high cost of titanium and columbium and a required surplus of these hardening elements in the presence of carbon for achieving the precipitation hardening effect. A considerable amount of the titanium and columbium primarily functions to tie up carbon in the steel. It is an excess over and above the amount required for tying up the carbon which contributes to the precipitation hardening reaction. The titanium also has a thirst for nitrogen and is subject to become tied up in the form of nitrides with consequent impairment of the hardening effect.

It has heretofore been the practice in producing and treating certain grades of stainless steel to introduce aluminum into the steel melt for purposes of deoxidation. Likewise, through alloying the element with other elements present, it is possible to produce stainless steel having an aluminum-containing surface oxide film which is resistant to heat and corrosion. The inclusion of aluminum in certain stainless steels is known to impart high temperature properties which, depending upon degree, have justied production from the metal of such articles as heat engine valves, turbine blades, parts i'or high temperature chemical equipment, and the like.

With reference tomany of the stainless steels, aluminum is said b v previous investigators to exert an impairing effect upon hardenability. I find that this by i'ar is not true of all stainless steels; instead, aluminum when properlyA correlated with other elements of the steels offers great benets as a relatively cheap precipitation hardening agent which, i'or any given amount included, is substantially fully effective in the hardening reaction despite the presence of carbon and small or any more than incidental amounts of nitrogen.

An outstanding object of my invention accordingly is the provision of a precipitation hardening chromium-nickel-aluminum stainless steel which is hardenable from an annealed con dition, highly suited to working, forming and fabrication, is readily hardened by low temless steel to a critical alloy content through the close correlation of chromium, nickel, aluminum and carbon therein, the resulting steel in proper condition of heat treatment has desirable working and forming properties. It may be hardened by direct low temperature heat exchange treatment without harmful heat eilects. By hardening, the mechanical strength is increased.

The more general practice of my invention embodies the provision of stainless steel in which the chromium and nickel contents are in substantial accord with the abscissa and ordinate of any given point of area ABCD in the accompanying diagram, and in which there are anywhere from approximately 0.02% to 0.12% carbon, aluminum from about 0.50% to 2.50%, from incidental amounts up to approximately 2.0% silicon, with or without molybdenum ranging up to about 3.0% illustratively to enhance corrosion resistance of the steel and the remainder substantially all iron. In this, however, should the carbon, aluminum, silicon or manganese content be somewhat different from the amount upon which the diagram is based (the basis of the diagram being amounts of actual chromium and nickel as called for, about 0.05% to 0.07% carbon, 0.60% to 1.0% aluminum, incidental amounts of manganese and silicon up to 1.0% each, sulphur and phosphorus up to 0.040% each, and the remainder substantially all iron) or should molybdenum be used, I find it preferable to modify the chromium or nickel content of the steel, or both of the contents as the case may be, so as to achieve chromium-like and nickel-like components in the steel which are substantially equivalent respectively in ferrite and austenite forming relation to those amounts of chromium and nickel and of the other elements represented in the diagram. For example, I often replace a part of the chromium called for by the diagram with a quantity of aluminum, silicon or molybdenum, the replacement being approximately on a 1 to 1 ratio with respect to chromium and for such purpose as maintaining a desired relation between the austenite and ferrite forming components as would be achieved by rigid adherence to the diagram. Similarly, I occasionally add several or all of the replacement elements in partial substituticn for chromium. The practice of substitution I find is particularly advantageous where either the aluminum or silicon is to exceed about 1.0%, or where molybdenum is a constituent of the steel. Following substitution, the actual chromium content of the steel may at times be somewhat below the amounts prescribed by area ABCD in the diagram and still, in view of the effect of the substituted element or elements, be in substantial accord with the area.

Should the permissible quantity of carbon exceed about 0.07%, or should more than small amounts of manganese be present (say for example amounts in excess of about 1.0%) I usually employ a proportionately decreased quantity of nickel in the steel as compared with the accompanying diagram. The actual nickel content of the steel importantly is more than about 3.5% after substitution. Where the permissible quantity of carbon is on the low side, say below about Iii) 0.05%, I usually increase the nickel contents as compared with the diagram, or even at times insteadincrease the manganese content. For each part of nickel, I add about 2 parts manganese or on the order of about 1/ao to 1/so part carbon as the substantial equivalent. The actual nickel content of the steel, after partial substitution, consequently may on occasions fall over considerably outside those amounts prescribed by area. ABCD in the diagram or remain inside and still, in view of the substantial element or elements and the contributed effect thereof, be in substantial accord with the area.

Where desired the steel contains such additional elements as sulphur and/or selenium 1n amounts sufficient to enhance free-machining properties. The value of such elements is realized especially in machining the steel in the annealed condition.

My steel in preferred composition contains chromium and nickel in amounts substantially in accord with the abscissa and ordinate respectively of any given point substantially falling Within area abcd in the accompanying diagram, the amounts advantageously being approximately 15.5% to 16.5% actual chromium and about 6.5% to 7.25% actual nickel as represented by the area abcd. The steel, also preferably contains, directly on the basis of the diagram, aluminum in amounts between about 0.60% and 1.0%, carbon within the approximate range of 0.05% to 0.07%, and the remainder substantially all iron. There are preferably small amounts of such elements as manganese, silicon, sulphur, and phosphorus in the steel, as for example manganese and silicon each not exceeding about 1.0% and sulphur and phosphorus each up to about 0.040%.

I heat the steel at a temperature not lower than about 1300 F. and extending up to about 1800 F. or more, and preferably between about 1500 F. to 1600 F., for such period of time as to put the aluminum and carbon in solid solution and accordingly provide an unbalanced austenitic structure which is transformable above usual room temperature. This is in the nature of an annealing treatment. In accomplishing the annealing treatment, I usually employ a heat treating furnace to hold the metal at temperature for the necessary period of time. The holding time is not too critical. About one-half hour usually is quite satisfactory from the standpoint of economy and of ensuring adequate solubility of the aluminum and carbon. I then quench the metal from the treating temperature as in air, oil or water, conveniently to around room temperature. By quenching, transformation of the austenitic constituent occurs without substantial precipitation of any aluminum compound. As quenched, the steel is reasonably ductile and has a hardness in or near the range of C-24 to C-30 Rockwell. It is formabl'e, as by cold reduction, and is machinable, one or more of which properties contribute in making it possible to fabricate the steel at this point into any of a wide variety of products susceptible to subsequent hardening.

The annealed and transformed steel is converted into sheet, strip, plates, bars and wire. Where desired, I fashion the chromium-nickelaluminum steel into more intricate shapes, as for example structural members for airplanes inclusive of those parts requiring great strength from the standpoints of yield strength in tension and compression coupled with corrosion resistance and toughness. Also the steel is fashioned into cold-headed bolts, rivets and screws, as for exam- 5 ple those eventually to have hardened Shanks, valves and valve seats, die blocks, fixtures and trim, and surgical instruments including those in the field of dentistry. For these products, I take advantage of the working and fabricating propertles of the metal, such as cold-forming, upsetting, drawing, punching, stamping, machining or l two hours or more withsatisfactory results. This treatment serves to precipitate an aluminum compound and impart a material gain in hardness and strength to the metal. The hardness upon quenching the metal as in air, oil or water from the hardening temperature comes within the approximate range of C-39 to C-46 Rockwell. though an aluminum compound is precipitated during my hardening treatment, the exact changes which take place are not so clear. It would seem that no changes in volume of the metal are to be noted of the hardening reaction. The reaction may, itis believed, involve a rearrangement or ordering of the lattice structure of a precipitated aluminum-nickel compound with the lattice structure of the matrix which thereafter exerts an interference hardening effect. This belief, however, is not fully confirmed and, therefore, I do not wish to be bound by the same.

By virtue of the low temperature hardening heat treatment to which the chromium-nickelaluminum stainless steel responds,.the hardened metal emerges substantially free of warping and heat scale. Thereis no appreciable change in volume of the iinally hardened metal as compared with volume in the prehardened state, which feature permits the carrying over of dimensions established to within close tolerances before the hardening treatment. In the hardened condi-, tion, the steel and products of the steel have excellent ultimate and yield strength in both tension and compression and consequently are reasonably free of directionality. They have a reasonable amount of ductility. Moreover, they possess excellent resistance t9 corrosion.

As illustrative of certain properties of a chromium-nickel-aluminum stainless steel provided and treated in accordance with my invention, I refer to Table I below which presents approximate physical values with reference to a steel containing approximately 0.062% carbon, 16.5% chromium, '7.00% nickel, 0.69% aluminum and the remainder substantially all iron with small amounts of manganese, silicon, sulphur and phosphorus. All values tabulated for the annealed condition resulted from one-half hour heating of the steel at about 1600 F. followed by a water quench, and those for the hardened condition from heating the steel at 900 F. for about one hour and quenching in air.

The close correlation of aluminum, a compara- 5 temperatures.

tively cheap material, with other elements in the steel, inclusive of nickel which is present in relatively small amounts, accordingly enables a commercially valuable precipitation hardening steel which becomes effectively hardened, with propel treatment from a soft workable or worked condition. It will further be appreciated that the aluminum contributes to heat resistance and to the prevention of heat scale formation. as during the hardening heat and in the resulting hardened corrosion resistant products.

Thus, it will be seen that there is provided in this invention a stainless steel and a related method, in which the various objects hereinbefore noted together 'with many thoroughly practical advantages are successfully achieved. It will be seen that the method enables the expedient production of hardened products with a minimum of treatment. It will further be seen that 20 the method makes possible the provision of wrought or cast chromium-nickel-aluminum stainless steel subjected to a number of forming, machining or fabricating operations and effectively and reliably hardened by treatment at low Also, it will be noted that the hardened products have many highly desirable physical properties including strength and corrosion resistance.

As many possible embodiments may be made of my invention and as many changes may be made in the embodiment hereinbefore set forth, it is to be understood that all matter described herein is to be interpreted as illustrative and not as a limitation.

Iolaim:

1. A chromium-nickel-aluminum stainless steel susceptible to annealing and quenching through phase transformation to a substantially fully aluminum-soluble condition and thereafter to pre- 40 cipitation hardening, said steel containing chromium and nickel in amounts substantially in accordance with area ABCD in the accompanying diagram, carbon between about 0.02% and 0.12%, about 0.50% to 2.50%aluminum, from incidental amounts up to approximately 8.0% manganese, from incidental amounts up torabo'ut 2.0% silicon, and the remainder substantially all iron.

2. A chromium-nickel-aluminum stainless steel susceptible to annealing and quenching through 0 phase transformation to a substantially fully aluminum-soluble condition and thereafter to precipitation hardening, said steel containing chromium and nickel in amounts substantially in accordance with area ABCD in the accompanying diagram, carbon between about 0.02% and 0.12%, about 0.50% to 2.50% aluminum, from incidental amounts up to approximately 8.0% manganese, from incidental amounts up to about 2.0% silicon, at least one element of the group consisting Gi of sulphur and selenium suiiicient for free-machining properties after annealing, and the remainder substntially all iron.

3. A chromium-nickel-aluminum stainless steel susceptible to annealing and quenching through phase transformation to a substantially fully aluminum-soluble condition and thereafter to precipitation hardening, said steel containing chromium and nickel in amounts substantialliy in accordance with area ABCD in the accompanying diagram, carbon between about 0.02% and 0.12%, about 0.50% to 2.50% aluminum, fractional percentages up to about 3.0% molybdenum, manganese from incidental amounts up to approximate- 7,-, ly 8.0%, from incidental amounts up to about 2.0% silicon, and the remainder substantially all iron.

4. A chromium-nlckel-aluminum stainless steel susceptible to annealing and quenching through phase transformation to a substantially fully aluminum-soluble condition, said steel containing about 15.5% to 16.5% chromium, about 6.5% to 7.25% nickel, aluminum between about 0.60% and 1.0%, about 0.05% to 0.07% carbon, and the remainder substantially all iron, and the steel being aluminum-precipitation hardenable by heat treatment after the solution annealing treatment and quenching.

54. In a method of aluminum-precipitation hardening chromium-nickel stainless steel, the art which includes providing a steel containing about 15.5% to 16.5% chromium, with about 6.5% to 7.25% nickel, aluminum in amounts between about 0.60% and 1.0%, and about 0.05% to 0.07% carbon, and the remainder substantially all iron; then annealing the steel at about 1300Jv F. to 18003 F. and quenching the same thus achieving phase transformation and a substantially fully-aluminum-solufble structure; and heating the transformed metal between about 750 F. and 1000 F. for a suflicient period of time to precipitate an aluminum compound and obtain a substantial increase in the alloy hard- IleSS. C

6. In the production of aluminum-precipitatransformation and a substantially fully alu-t minum-soluble structure; fabricating the articles and products of the transformed steel; and heating the fabricated articles and products between 750 F. and 1000 F. for a sufficient period of time to precipitate an aluminum compound and obtain a substantial increase in the alloy hardness.

7. A chromium-nickel stainless steel fabricated article aluminum-precipitation hardened by a single heat treatment from the annealed condition, containing about 15.5% to 16.5% chromium, about 6.5% to 7.25% nickel, aluminum between about 0.60% to 1.0%, about 0.05% to 0.07% carbon and the remainder substantially al1 iron, said aluminum being finely precipitated as aluminum compound within the matrix of the steel to give substantial hardness.

8. A chromium-nickel stainless steel aluminum-precipitation hardened by a single heat treatment from the annealed condition, containing chromium and nickel in amounts substantially in accordance with area ABCD in the accompanying diagram, carbon between about 0,02% and 0.12%, about 0.50% to 2.50% aluminum, from incidental amounts up to approximately 8.0% manganese, from incidental amounts up to about 2.0% silicon, and the remainder substantially all iron.

9. A chromium-nickel stainless steel aluminum-precipitation hardened by a single heat treatment from the annealed condition, containing chromium and nickel in amounts substantially in accordance with area ABCD in the accompanying diagram, carbon between about 0.02% and 0.12%, about 0.50% to 2.50% aluminum, fractional percentages up to about 3.0% 5 molybdenum, manganese from incidental amounts up to approximately 8.0%, from incidental amounts up to 2.0% silicon, and the remainder substantially all iron.

10. A chromium-nickel-aluminum stainless steel susceptible to annealing and quenching through phase transformation to a substantially fully aluminum soluble condition and thereafter to precipitation hardening, said steel containing: "the chromium-like components aluminum in the amount of about 0.5% to 2.50%, silicon from incidental amounts up to about 2.0% based on total content of the steel, and the remainder substantially all chromium, said components in total meeting the terms for chromium of area ABCD of the accompanying diagram, said aluminum and silicon serving as a substitute for chromium on about a 1 to 1 ratio; the

` nickel-like components carbon in the amount of about 0.02% to 0.12%, manganese from incidental amounts up to about 8.0%, and the remainder substantially all nickel in actual amount not less than about 3.5%, said nickel-like components in total meeting the terms for nickel'of area ABCD of the accompanying diagram, and said carbon and manganese serving as a substitute for nickel on the ratios of about 1/20 to 1/30 part carbon to 1, and approximately 2 parts manganese to 1; and the remainder substantially all iron.

11. A chromium-nickel stainless steel alu` minum precipitation hardened by a single heat treatment from the annealed condition, said steel containing; the chromium-like components aluminum inthe amount of about 0.50% to 2.50%. silicon from incidental amounts up to about 2.0% based on total content of the steel, and the remainder substantially all chromium, said components in total meeting the terms for chromium of area ABCD of the accompanying diagram, said 45 aluminum and silicon serving as a substitute for chromium on about a l to 1 ratio; the nickellike components carbon in the amount of about 0.02% to 0.12%, manganese from incidental amounts up to about 8.0%, and the remainder 50 substantially all nickel in actual Iamount not less than about 3.5% meeting the terms for nickel of area ABCD of the accompanying diagram, said carbon and manganese serving 'as a substitute for nickel on the ratios of about 1/zo to 1/30 part 55 carbon to 1, and approximately 2 parts manganese to 1; and the remainder substantially all iron, said aluminum being precipitated to give substantial hardness.

GEORGE N. GOLLER.

REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS 

11. A CHROMIUM-NICKEL STAINLESS STEEL ALUMINUM PRECIPITATION HARDENED BY A SINGLE HEAT TREATMENT FROM THE ANNEALED CONDITION, SAID STEEL CONTAINING; THE CHROMIUM-LIKE COMPONENTS ALUMINUM IN THE AMOUNT OF ABOUT 0.50% TO 2.50%, SILICON FROM INCIDENTAL AMOUNTS UP TO ABOUT 2.0% BASED ON TOTAL CONTENT OF THE STEEL, AND THE REMAINDER SUBSTANTIALLY ALL CHROMIUM, SAID COMPONENTS IN TOTAL MEETING THE TERMS OF CHROMIUM OF AREA ABCD OF THE ACCOMPANYING DIAGRAM, SAID ALUMINUM AND SILICON SERVING AS A SUBSTITUTE FOR CHROMIUM ON ABOUT A 1 TO 1 RATIO; THE NICKELLIKE COMPONENTS IN THE AMOUNT OF ABOUT 0.02% TO 0.12%, MANGANESE FROM INCIDENTAL AMOUNTS UP TO ABOUT 8.0%, AND THE REMAINDER SUBSTANTIALLY ALL NICKEL IN ACTUAL AMOUNT NOT LESS THAN ABOUT 3.5% MEETING THE TERMS FOR NICKEL OF AREA ABCD OF THE ACCOMPANYING DIAGRAM, SAID CARBON AND MANGANESE SERVING AS A SUBSTITUTE FOR NICKEL ON THE RATIOS OF ABOUT 1/20 TO 1/30 PART CARBON TO 1, AND APPROXIMATELY 2 PARTS MANGANESE TO 1; AND THE REMAINDER SUBSTANTIALLY ALL IRON, SAID ALUMINUM BEING PRECIPITATED TO GIVE SUBSTANTIAL HARDNESS. 